Portable wireless communication devices such as pagers, mobile (cellular) telephones, Personal Digital Assistants (PDAs) are all required to consume as little power as possible during operation as each is battery operated. Because such devices use batteries which have a limited lifetime, it is desirable to extend the battery lifetime as far as possible before requiring replacement or recharge. As each of these portable, wireless communication devices generally includes an LCD, the power consumed in the LCD has often been evaluated.
Graphical LCD displays such as those used on portable, wireless communication devices, are also referred to as multiplex ed LCDs. Simple non-multiplexed LCDs have separate external connections for each and every segment plus a common back plane. Multiplexed LCDs have segments arranged at intersections of an x-y grid. The x-y grid is formed by an array of the rows and columns used to implement the LCD display. The arrangement of such segments is done to greatly reduce the number of external connections and increase potential display density.
LCD controllers are often used to control the operation of the LCD display and supply the display drivers with data. There are many different configurations of LCD controllers in existence. Currently, most LCD controllers require pixel-based graphical data to be fetched from a memory circuit in a repetitive and cyclical fashion. In fetching the pixel-based graphical data, enough data for the whole display is retrieved. Furthermore, to prevent flickering of the LCD display, the data must be provided to the LCD display by the LCD controller and refreshed at a predetermined, recommended rate.
For an LCD display requiring substantial amounts of data, and requiring the data be fetched each time in order to provide the display in a repetitive and cyclical fashion, the data access rate required to support refresh dictates that the data be accessed at higher frequencies than the common crystal rates of 38.4 kilohertz for a paging crystal, or o 32.768 kilohertz a real-time clock (RTC) crystal. The required higher frequencies are synthesized using phase lock loop circuits, or other high frequency clock source. Both the use of phase lock loop circuitry and the retrieval of a substantial amount of data from an external memory increase power consumption requirements of a system, and also increase the overhead cost of a system implementing the circuit as additional circuitry and control information are required. Additionally, a substantial amount of power is consumed by requiring Pixel-based graphical data to be accessed from a memory which is external to the LCD controller and the LCD display. Both the interface between the external memory and the LCD controller as well as the interface between the LCD controller and the LCD display require a significant amount of power to transfer the required data.
Additionally, as previously mentioned, current LCD controllers transfer sufficient data to refresh the entire display irrespective of the amount of information actually being displayed. Such a methodology is efficient if most or all of the display area is used each time the display is activated or enabled. However, when only a portion of the display area is activated or enabled, such methodology proves to be inefficient and consumes power in an inefficient manner. An example of such inefficient power consumption requirement is illustrated when a paging function is added to a digital organizer having a graphical display. Here one line of text might be sufficient to display the contents of the paging operation. The same inefficiency is true of a global positioning system (GPS) where a subset of the data might be all that is required for a substantial percentage of the time when the graphical display is enabled. It is desirable for LCD controllers to compensate for the use of a small portion of an LCD display and avoid using the entire LCD display when only a small percentage of the data space is needed. Additionally, it is desirable to only refresh the desired display area which is a subset of the entire display. It is further desirable to reduce the power required within an LCD display system by providing data only as needed.